1. Field of the Invention
The present invention relates to manufacture of pipes by roll bending a metal sheet, and more particularly to manufacture of elongated pipes having a small diameter.
2. Background Information
Manufacturing methods of pipes include a system that continuously manufactures pipes by electric welding and a system that manufactures them by roll bending with a use of a bending roll apparatus.
In the former, a manufacturing apparatus has a large scale, is expensive and is suitable for mass production. In the latter, in contrast, a manufacturing apparatus is economical but is not suitable for mass production.
To manufacture a thin elongated pipe, the conventional manufacturing system by electric welding can shape and weld a steel sheet having a thickness of at least 0.001 times a pipe diameter but this system is generally unsuitable for sheets having a smaller thickness than the above.
In the case of a system using the bending roll apparatus, on the other hand, shaping of a thin pipe is easy. When an elongated pipe having a relatively large length to diameter ratio is machined by using the bending roll apparatus, a core roll itself undergoes deflection in a longitudinal direction, so that deflection becomes great at a portion near the center of a roll and a roll push amount at this portion becomes small. Consequently, when a metal sheet is shaped into a pipe form, a gap develops at the center in the longitudinal direction of a product pipe as shown in FIG. 18 and the pipe becomes barrel-like. This tendency becomes remarkable when an elongated pipe is shaped. Therefore, when a pipe is produced by use of a steel material, it has been difficult, in many cases, to shape, by this method, an elongated pipe having a length that is at least about 10 times the pipe diameter.
To solve the problem of such barrel-like deformation, various attempts have been made by providing a crown to a core roll or by providing a backup roll to a push roll and intentionally causing deflection in a transverse direction so that the core roll and the push roll can keep a predetermined spacing, as typified by a bending roll apparatus described in Japanese Patent Publication No. 8-117869.
However, the prior art technologies for eliminating the barrel-like deformation by providing the crown to the core roll or by providing the backup roll to the push roll and intentionally causing deflection in the transverse direction so as to allow the core roll and push roll to maintain a predetermined gap cannot sufficiently compensate when an elongated pipe having a small diameter, as a work, is shaped because deflection of the core roll becomes excessively great. For example, an elongated pipe having a diameter of 50 mm and a length of 2,000 mm cannot be shaped by roll bending.
Japanese Patent Publication No. 9-70622 describes a roll bending method which can be used to prevent deflection of the core roll. When the metal sheet to be worked does not exist on the surface of the core roll on the side of a multi-diameter push roll having two kinds of diameters of a large arc portion and a small arc portion, the apparatus executes a push work on the large arc side. When the metal sheet exists on the roll surface, it executes the push work on the small arc side. This method applies a predetermined pressure to the core roll by such construction, and attempts to prevent the barrel-like deformation.
However, the technology described in this Japanese Patent Publication No. 9-70622 generally works a metal sheet having a length below the circumferential length of the core roll and cannot easily register the distal end of the metal sheet to be worked with a step portion of the push roll particularly when a small diameter elongated pipe of a high tensile strength metal sheet having a high elastic recovery ratio is manufactured. A manufacturing step of press-shaping the pipe by a shrink press after the metal sheet is shaped into the pipe and correcting the barrel-like deformation may be added.
Further, even when this shrink press step is employed, a shrink mold must be formed for each of the small diameter pipes and this invites an increase in the production cost.